Organic solvents that evaporate during the application of coatings and inks contribute significantly to a wide variety of air quality problems. Sunlight is one of the key factors that cause these chemicals to react with each other, thus the term "photochemical smog". These polluting organic solvents are most commonly referred to as Volatile Organic Compounds (VOC's). Ozone is formed by photochemical reactions between nitrogen oxides from fuel combustion and VOC's. Elevated ozone concentrations reduce lung function, aggravate allergies, damage vegetation and cause eye irritations.
Consequently, the Environmental Protection Agency (EPA) and local Air Quality Management Districts have stepped up their efforts to regulate the amount of VOC's. Although coating and ink formulations represent only a small segment of the total market, any reduction of VOC's from these products will help achieve acceptable environmental conditions, and help prepare the industry for the probability of more stringent air pollution regulations in the future.
U.S. Pat. Nos. 4,704,309 and 4,738,785 relate to aqueous ink compositions containing a water dispersible sulfo-polyester. Inks containing such water dispersible sulfo-polyesters have many desirable properties such as good adhesion to a variety of substrates and a wide viscosity range. However, such inks display poor water resistance and poor block resistance on certain substrates.
U.S. Pat. Nos. 4,921,899 and 5,075,364 disclose ink compositions containing water dispersible sulfo-polyesters, acrylic polymer emulsions, surfactants and volatile organic compounds. Inks containing the blend of these polymers exhibit improved block, alcohol and water resistance as compared to inks containing the water dispersible polyester alone. The presence of surfactants, however, in the ink formulations creates several problems related to ink stability, printing process and print quality of the ink film.
U.S. Pat. Nos. 4,996,252 and 5,039,339 disclose ink compositions containing blends of water dispersible sulfo-polyesters and acrylic polymer emulsions. The acrylic polymer emulsions have molecular weights of greater than 200,000. In contrast, the blends of the present invention do not contain acrylic polymer emulsions.
Copending commonly assigned U.S. Pat. application Nos. 638,929 and 638,912 disclose processes for preparing water-dispersible sulfo-polyesters and acrylic resin blends. However, the acrylic resins used in the preparation of the blends require volatile organic compounds in the range of 7-15 weight percent. Without such volatile compounds, the blends gel upon cooling.
The present inventor has unexpectedly developed a process for preparing blends of water dispersible sulfo-polyesters and acrylic resins which do not contain volatile organic compounds and surfactants. Moreover, the present inventor has unexpectedly determined that a correlation exists between the molecular weight and the acid number of the acrylic resin. The blends produced by the process of the present invention provide ink compositions with good water resistance and good block resistance.